International Communications in Heat and Mass Transfer 52 (2014) 51–55

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International Communications in Heat and Mass Transfer

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Experimental investigation of vortex tube using natural substances☆

N. Agrawal ⁎, S.S. Naik, Y.P. GawaleDepartment of Mechanical Engineering, Dr. Babasaheb Ambedkar Technological University, Lonere 402103, Dist.: Raigad, Maharashtra, India

☆ Communicated by A.R. Balakrishnan and T. Basak.⁎ Corresponding author.

E-mail addresses:neeraj.titan@gmail.com (N. Agrawalgawaleyogesh@gmail.com (Y.P. Gawale).

0735-1933/$ – see front matter © 2014 Elsevier Ltd. All rihttp://dx.doi.org/10.1016/j.icheatmasstransfer.2014.01.00

a b s t r a c t

a r t i c l e i n f o

Available online 11 January 2014

Keywords:Vortex tubeCarbon dioxideOptimum cold mass fractionIsentropic efficiency

An experimental investigation is carried out on Ranque–Hilsch vortex tube (RHVT). Influential parameters suchas L/D ratio, cold mass fraction, inlet pressure etc. are investigated. Further, three different working media (air,nitrogen and carbon dioxide) are also tested. An in-house facility is developed to test the vortex tubes. A valueof cold mass fraction is observed at which vortex tube performs optimally at the given pressure and L/D ratio.It is found that vortex tube performs better with carbon dioxide as working fluid.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The vortex tube is a simple device used in industry for generation ofcold and hot air streams from a single compressed air supply; it has nomoving mechanical parts. It converts a gas flow initially homogeneousin temperature, into two separate flows of differing temperatures. Thevortex tube contains the following parts: an inlet nozzle, a vortex cham-ber, a cold-end orifice, a hot end control valve and a tube. It separatescompressed gas stream into a low total temperature region and a highone by energy separation [1].

When high-pressure gas is tangentially injected into the vortexchamber via the inlet nozzles, a swirling flow is created inside the vor-tex chamber. When the gas swirls to the center of the chamber, it is ex-panded and cooled. In the vortex chamber, part of the gas swirls to thehot end, and another part exits via the cold exhaust directly. The part ofthe gas in the vortex tube reverses for axial component of the velocityand move from the hot end to the cold end. At the hot exhaust, thegas escapes with a higher temperature, while at the cold exhaust, thegas has a lower temperature compared to the inlet temperature [1].

Themechanismof energy separation in the vortex tube is an impres-sive and indefinite phenomenon; some works have been published toexplain this phenomenon based on physical laws such as conservationof mass and momentum, first and second laws of thermodynamics.Therefore, several different hypotheses have been reported to describethe energy separation phenomenon.

Xue et al. [2] reported that the temperature separation in a vortextube depends on factors such as pressure gradient, viscosity, flow struc-ture in the tube and acoustic streaming. Lewins and Bejan [3] suggestedthat the angular velocity gradients in the radial direction give rise to fric-tional coupling between different layers of the rotating flow resulting in

), ssnaik@dbatu.ac.in (S.S. Naik),

ghts reserved.9

a migration of energy via shear work from the inner layers to the outerlayers. Saidi and Yazdi [4] had used a thermodynamic model to investi-gate vortex tube energy separation. Parametric study and dimensionoptimization have been carried out using exergy analysis.

Saidi and Valipour [5] presented parametric study of vortex tubeusing air as a working medium. Eiamsa-ard [6] studied the effect ofthe snails with the inlet nozzle number of 1, 2, 3 and 4 nozzles and ob-served that it offers higher temperature separation and cooling efficien-cy in the RHVT as compared to the conventional tangential inlet nozzles(4 nozzles). Aydin et al. [7] and Markal et al. [8] experimentally investi-gated in their seminal studies the effects of conical valve angle on ther-mal energy separation by using a novel design cold end side ‘helicalswirl flow generator’ in a counter flow vortex tube. Xue and Arjomandi[9] investigated the effect of vortex angle on the performance. A smallervortex angle demonstrated a larger temperature difference and betterperformance for the heating efficiency of the vortex tube. However,small vortex angles resulted in better cooling efficiency only at lowervalues of input pressure. Eiamsa-ard et al. [10] investigated experimen-tally the effect of cooling of hot tube on the energy/temperature separa-tion in the RHVT. The results show that the cooling of a hot tube plays arole in the enhancement of the temperature reduction of the cold airand thus cooling efficiency of the RHVT. Wu et al. [11] designed threenovel vortex tubes and the performance is tested. A new nozzle withequal Mach number gradient and an intake flow passage with equalflowvelocitywere used in themodified vortex tube. The results indicatethat the cooling effect of the improved nozzle is about 2.2 °C lower thanthat of the nozzle with normal rectangle and even 5 °C lower than thatof the nozzle with Archimedes' spiral. Eiamsa-ard and Promvonge [12]reviewed on vortex tube and found that the optimum values for thecold orifice diameter (Dc/D), the angle of the control valve, the lengthof the vortex tube (L/D), and the diameter of the inlet nozzle (Din/D)are approximately Dc/D ≈ 0.5, L/D ≈ 20, and (Din/D) ≈ 0.33, respec-tively which may be used as guidelines to design the vortex tube.

Most of the studies are carried out with air as the working fluid invortex tube. Studies with other natural substances such as nitrogen

9

φ 9

φ2.5

φ4

12

φ10

φ6

6

1

2

3 4 5

2

7

812

7

6

Schematic of experim

Assembly of Vo

Conical valve

Fig. 1. Schematic of experimental setup a

Nomenclature

min Mass flow rare at supply nozzle (kg s−1)mc Mass flow rare at cold end (kg s−1)mh Mass flow rare hot end (kg s−1)μc Cold mass fraction (%)Tin Temperature at supply nozzle (°C)Tc Temperature at cold end (°C)Th Temperature at hot end (°C)ΔTc Cold end temperature difference (°C)ΔTh Hot end temperature difference (°C)L Length of vortex tube (mm)D Diameter of vortex tube (mm)Dc Cold end orifice diameter (mm)η Isentropic efficiency (%)Qc Cooling effect (W)W Compressor work (W)

52 N. Agrawal et al. / International Communications in Heat and Mass Transfer 52 (2014) 51–55

and carbon dioxide are relatively less in open literature. Aydin and Baki[13] tested three different gases (air, oxygen and nitrogen). It was ob-served that nitrogen provides higher temperature differences as com-pared to air and oxygen. Sarkar [14] suggested that use of vortex tubeas an expansion valve is more effective for higher temperature lift interms of higher COP improvement and lower optimum discharge pres-sure for CO2 transcritical refrigeration systems. In the present work, theoptimization of the vortex tube is carried out experimentally using nat-ural working substance; air, nitrogen and carbon dioxide.

2. RHVT design and experimental test facility

Vortex tube is designed based on the following correlations for larg-er temperature difference [1].

Din=D≤ 0:2 ð1Þ

Dc=ND2in ≤ 2:3 ð2Þ

120 28

φ20

M6

M16

φ10

12

28

1

6

910

11 7

1 Compressor2 Pressure gauge3 Receiver4 Control valve5 Pressure regulator6 Rotameter7 Thermocouple8 Inlet nozzle9 Vortex tube10 Conical valve11 Hot end12 Cold end

ental setup

rtex Tube

Hot end

nd geometrical detail of vortex tube.

15

20

25

30

0 20 40 60 80 100

Cold mass fraction (%)

Isen

trop

ic E

ffici

ency

(%

)

Fig. 2. Variation of isentropic efficiency with cold mass fraction with uncertainty bars.

12

15

18

21

24

27

0 10 20 30 40 50 60 70 80 90 100

% Cold mass fraction

Ti -

Tc

(°C

)

Dc = 3 mm

Dc = 4 mm

Dc = 5 mm

Gas - AirPr = 3 barL/D = 17.5

Fig. 4. Effect of orifice diameter on cold end temperature drop.

53N. Agrawal et al. / International Communications in Heat and Mass Transfer 52 (2014) 51–55

DcbD−2Din ð3Þ

Three different single nozzle vortex tubes of the length 125 mm,175 mm and 225 mm of L/D = 12.5, 17.5 and 22.5 are tested with thetube diameter as 10 mm and inlet nozzle diameter as 2 mm. Cold enddiameter and cone angle of hot valve was taken as 3 mm, 4 mm &5 mm and 45°, respectively.

An in-house experimental test facility is developed as shown in Fig. 1along with the geometrical details. Compressed air from the compressor(1) passes through the control valve (4) and pressure regulator filter sec-tion (5) and enters in the vortex tube (9) tangentially. The compressedair expands in the vortex tube and gets divided in to cold and hotstreams. The cold air leaves the cold end orifice (12) near the inlet nozzle(8) while the hot air discharges at the periphery at the other end of thetube i.e. hot end (11). The control valve (needle valve) controls the flowrate of thehot air (10). Two rotameters (6)measures themassflow ratesof the supply and cold air. T type thermocouples (7) are used tomeasurethe temperature of the leaving cold and hot air in the vortex tube. Thepressure of inlet gas is measured by pressure gauge (2).

3. Data reduction

In the present study the parameters used to evaluate performance ofvortex tube are defines as follows:

Cold mass fraction

μc ¼mc

min: ð4Þ

Cold temperature drop and hot temperature difference

ΔTc ¼ Tin−Tc and ΔTh ¼ Th−Tin: ð5Þ

15

18

21

24

27

30

0 20 40 60 80 100

% Cold mass Fraction

Isen

trop

ic E

ffici

ency

(%

)

L/D=12.5

L/D=17.5

L/D=22.5

Gas = AirPr=4 bar

Fig. 3.Variation of cold end temperature dropwith coldmass fraction at various L/D ratios.

Cooling efficiency of vortex tube using the principle of adiabatic ex-pansion of ideal gas

ηis ¼Tin−Tc

Tin 1− PatmPin

� � γ−1=γ

� �" # : ð6Þ

Coefficient of performance using the principle of isentropic expan-sion of ideal gas

COP ¼ μCp Tin−Tcð Þγ�

γ−1

� �RTin

PatmPin

� � γ−1=γ

� �−1

" # ð7Þ

Uncertainty is carried for Isentropic efficiency which has functionaldependency at inlet and cold end temperature given as:

U ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

∂η∂Ti

w1

� �2

þ ∂η∂Tc

w1

� �2" #vuut : ð8Þ

4. Results and discussions

Experiments are conducted on three different vortex tubes with L/Dratios; 12.5, 17.5 and 22.5. Inlet pressure is also varied from 3 to 5 bar inthe increment of 1 bar. For the testing, coldmass fraction is varied in therange of 10 to 90% with the step size of 10%. Three different gases— air,nitrogen and CO2 — are tested.

Initially uncertainty analysis was carried out for the test data and themaximum uncertainty in isentropic efficiency was observed in therange of 4 to 6% (Fig. 2). The effect of variation of cold end temperaturedrop (Ti − Tc) with the cold mass fraction at various L/D ratio is shownin Fig. 3. The cold mass fraction is varied by opening the needle valvegradually. The supply pressure of air was maintained at 4 bar. It canbe seen that for each L/D ratio, initially cold end temperature drop

15

18

21

24

27

30

33

0 20 40 60 80 100

% cold mass fraction

Ti -

Tc

(0 C)

3 bar4 bar5 bar

Gas -AirL/D = 17.5

Fig. 5. Effect of pressure on cold end temperature drop.

Table 1Typical results (working gas = air, working pressure = 4 bar, coldmass fraction =60%).

Performance parameters L/D ratio

12.5 17.5 22.5

Cold end temperature (°C) 27 29 24Isentropic efficiency (%) 27.2 29.2 24.3COP 0.110 0.118 0.098

54 N. Agrawal et al. / International Communications in Heat and Mass Transfer 52 (2014) 51–55

increases to maximum at an optimum value of cold mass fraction of60%. Maximum value of cold end temperature drop of 29 °C is obtainedfor L/D ratio 17.5 at 60% cold mass fraction while with L/D ratio of 12.5and 22.5, maximum cold end temperature drop values are about 26 °Cand 24 °C, respectively. This may due to incomplete separation of thecold air and hot air streams beyond a certain L/D ratio for a specific vor-tex tube; cold streammixes with hot stream resulting drop in cold endtemperature drop. In a vortex tubewith lower L/D ratio, the energy sep-aration may not take place properly due to less time available.

The effect of cold mass fraction of air on isentropic efficiency of vor-tex tube at various L/D ratio was also investigated. Maximum isentropicefficiency is obtained as 29.2% with L/D ratio 17.5. Similar to cold endtemperature drop, isentropic efficiency was observed maximum atcold mass fraction 60%.

The effect of cold mass fraction on COP was also studied at different(L/D) ratios. Vortex tube with L/D ratio 17.5 gives highest COP of 0.13when cold mass fraction is 80%. COP represents the amount of energyseparated inside a vortex tube, for this reason COP depends on L/Dratio of tube at a given coldmass fraction. Further, reduced temperatureseparation in shorter and longer tube (than the one with L/D ratio of17.5) offer lowCOP. It is seen that theCOP increaseswith coldmass frac-tion and is maximum at 80% cold mass fraction.

The effect of cold orifice diameter on cold end temperature drop isshown in Fig. 4. Results show that highest temperature drop is obtainedwith cold orifice diameter of 4 mm. The maximum cold temperaturedrop obtained for the other two cold orifice diameters, 3 mm and5 mm are 23 °C and 24 °C respectively. This can be explained with thefact that larger orifice draws back hot gases towards cold end causingits mixing with cold gas giving rise to reduces temperature drops.Also, large cold orifice tends to draw air directly from the inlet chargeand yield weaker tangential velocities near the inlet region resultinglow energy separation. If the orifice diameter is too small, there is a sig-nificant pressure drop across the orifice leading to a higher back pres-sure in the vortex tube resulting in low energy separation.

Fig. 5 depicts the variation of the cold end temperature drop of theoptimized vortex tube (L/D = 17.5) with cold mass fraction at variousinlet pressures of air. It is shown that increasing the inlet pressure in-creases the cold end temperature drop up to cold mass fraction of 60%.The highest temperature drop measured is 32 °C at the inlet pressure5 bar while 29 °C and 26 °C temperature drop were obtained at 4 barand 3 bar pressure supply, respectively. The highest cold end tempera-ture drop at the respective operating pressures is seen at 60% cold massfraction. This is due to the reason that as the pressure increases it causes

13

16

19

22

25

1 2 3 4 5 6

Pressure (Bar)

Ti -

Tc

(o C) Nitrogen (L/D = 12.5)

Air (L/D =12.5

CO2 (L/D = 12.5)

CO2 (L/D = 17.5)

Air (L/D = 17.5)

Fig. 6. Comparison of cold end temperature drop of air, nitrogen and CO2.

flow velocity to increase which become responsible for energy separa-tion. Increase in cold mass fraction beyond 60% reduces the coldtemperature drop. This can be attributed to the fact that as cold massfraction is increased beyond this value (60%), some of the operatingfluid in which energy separation as already occurred (which is at rela-tively higher temperature) is also drawn at cold end. This will causemixing of hot and cold mass of working fluid leading to net reductionin cold temperature drop. Considering the fact the cold end temperaturedrop is at the maximum at 5 bar, it can be said that the chosen vortextube of L/D 17.5 is capable of causing full expansion of the workingme-dium air at this pressure. However, the same trendmay not be continu-ing at higher working pressures. Further, it can be inferred that if thecold mass fraction is made 100% the working fluid temperature at thecold endwould become same as its temperature at the inlet, i.e. causingthe cold end temperature drop to reduce to zero. Typical results withthe working media as air at working pressure 4 bar is shown in Table 1.

The cooling effect produced in the vortex tube depends on proper-ties of the working fluids viz. molecular weight, specific heat capacityratio and moisture content of gas. Experiments were performed withthree different working fluids as air, nitrogen and CO2. Fig. 6 showsthe cold end temperature dropwith supply pressure for chosenworkingmedia with cold mass fraction 30%. The result shows that CO2 produceshigher cold end temperature drop than air and nitrogen. At pressure of4 bar cold temperature drop for CO2, nitrogen and air are 23 °C, 18 °Cand 20 °C, respectively. The highest temperature drop is obtainedwith CO2 due to the higher molecular weight and lower gas constantof CO2 in comparison to other gases. Further, lower specific heat ratio(γ) of the CO2 may also contribute to get the maximum cold tempera-ture drop for CO2. The temperature drop increases directly with pres-sure since higher pressure is indicative of more work potential.However, it cannot be said with confidence that temperature drop willcontinue to increase at this rate beyond 5 bar pressure without moreexperimentation.

5. Conclusions

Performance evaluation of the Ranque–Hilsch vortex tube has beencarried out experimentally. There is a value of cold mass fraction atwhich vortex tube performs optimally at the given pressure and L/Dratio. The maximum cold temperature drop is also obtained at coldmass fraction/ratio of 60%. Further, it is found that the vortex tube ofL/D ratio 17.5 performs optimally. For a given L/D ratio, as the gas pres-sure increases, cold end temperature difference increases but the opti-mum value of cold mass fraction remains same. In the tested range,COP and cooling capacity both shows increasing trend with cold massfraction up to cold mass fraction of 60%.

It is also observed that the cooling effect produced by the vortex tubedepends on properties of the gas, molecular weight and specific heatratio. The vortex tubes perform better with carbon dioxide comparedto air and nitrogen owing to its high molecular weight and low specificheat ratio.

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